OSI Layer Model definition, all seven layers in detail and diagram

1. OSI Layer Model

2. Physical Layer
• The lowest layer of the OSI reference model.
• Responsible for the actual physical connection between the devices.
• Contains information in the form of bits.
• When receiving data, this layer will get the signal received and convert it into 0s and 1s
and send them to the Data Link layer, which will put the frame back together.
• Functions:
• Bit synchronization: The physical layer provides the synchronization of the bits by providing
a clock. This clock controls both sender and receiver thus providing synchronization at bit
level.
• Bit rate control: The Physical layer also defines the transmission rate i.e. the number of bits
sent per second.
• Physical topologies: Physical layer specifies the way in which the different, devices/nodes are
arranged in a network i.e. bus, star or mesh topology.
• Transmission mode: Physical layer also defines the way in which the data flows between the
two connected devices. The various transmission modes possible are: Simplex, half-duplex and
full-duplex.

3. Data Link Layer
• Responsible for the node to node delivery of the message.
• Main function is to make sure data transfer is error free from one node to another, over
the physical layer.
• When a packet arrives in a network, it is the responsibility of DLL to transmit it to the
Host using its MAC address.
• Data Link Layer is divided into two sub layers :
• Logical Link Control (LLC)
• Media Access Control (MAC)
• The packet received from Network layer is further divided into frames depending on the
frame size of NIC(Network Interface Card). DLL also encapsulates Sender and
Receiver’s MAC address in the header.
• The Receiver’s MAC address is obtained by placing an ARP(Address Resolution
Protocol) request onto the wire asking “Who has that IP address?” and the destination
host will reply with its MAC address.

4. Data Link Layer
• Functions
• The functions of the data Link layer are :
• Framing: Framing is a function of the data link layer. It provides a way for a sender to
transmit a set of bits that are meaningful to the receiver. This can be accomplished by attaching
special bit patterns to the beginning and end of the frame.
• Physical addressing: After creating frames, Data link layer adds physical addresses (MAC
address) of sender and/or receiver in the header of each frame.
• Error control: Data link layer provides the mechanism of error control in which it detects and
retransmits damaged or lost frames.
• Flow Control: The data rate must be constant on both sides else the data may get corrupted
thus , flow control coordinates that amount of data that can be sent before receiving
acknowledgement.
• Access control: When a single communication channel is shared by multiple devices, MAC
sub-layer of data link layer helps to determine which device has control over the channel at a
given time.

5. Network Layer
• Works for the transmission of data from one host to the other located in different
networks.
• It also takes care of packet routing i.e. selection of the shortest path to transmit
the packet, from the number of routes available.
• The sender & receiver’s IP address are placed in the header by network layer.
• Functions:
• Routing: The network layer protocols determine which route is suitable from source
to destination. This function of network layer is known as routing.
• Logical Addressing: In order to identify each device on internetwork uniquely,
network layer defines an addressing scheme. The sender & receiver’s IP address are
placed in the header by network layer. Such an address distinguishes each device
uniquely and universally.
• Segment in Network layer is referred as Packet.

6. Transport Layer
• Provides services to application layer and takes services from network layer.
• The data in the transport layer is referred to as Segments. It is responsible for the
End to End delivery of the complete message.
• Transport layer also provides the acknowledgment of the successful data
transmission and re-transmits the data if an error is found.
• Functions:
• Segmentation and Reassembly: This layer accepts the message from the (session)
layer , breaks the message into smaller units . Each of the segment produced has a
header associated with it. The transport layer at the destination station reassembles the
message.
• Service Point Addressing: In order to deliver the message to correct process,
transport layer header includes a type of address called service point address or port
address. Thus by specifying this address, transport layer makes sure that the message
is delivered to the correct process.

7. Transport Layer
• Services provided by transport layer :
• Connection Oriented Service: It is a three-phase process which include
– Connection Establishment
– Data Transfer
– Termination / disconnection
In this type of transmission, the receiving device sends an acknowledgment, back to
the source after a packet or group of packet is received. This type of transmission is
reliable and secure.
• Connection less service: It is a one phase process and includes Data Transfer. In this
type of transmission, the receiver does not acknowledge receipt of a packet. This
approach allows for much faster communication between devices. Connection
oriented Service is more reliable than connection less Service.
• Data in the Transport Layer is called as Segments.
• Transport Layer is called as Heart of OSI model.

8. Session Layer
• Responsible for establishment of connection, maintenance of sessions, authentication and
also ensures security.
• Functions:
• Session establishment, maintenance and termination: The layer allows the two processes to
establish, use and terminate a connection.
• Synchronization : This layer allows a process to add checkpoints which are considered as
synchronization points into the data. These synchronization point help to identify the error so
that the data is re-synchronized properly, and ends of the messages are not cut prematurely and
data loss is avoided.
• Dialog Controller : The session layer allows two systems to start communication with each
other in half-duplex or full-duplex.
• Let’s consider a scenario where a user wants to send a message through some Messenger
application running in his browser. The “Messenger” here acts as the application layer
which provides the user with an interface to create the data. This message or so-called
Data is compressed, encrypted (if any secure data) and converted into bits (0’s and 1’s) so
that it can be transmitted.

9. Presentation Layer
• Presentation layer is also called the Translation layer.
• The data from the application layer is extracted here and manipulated as per the
required format to transmit over the network.
• Functions:
• Translation : For example, ASCII to EBCDIC.
• Encryption/ Decryption : Data encryption translates the data into another form or
code. The encrypted data is known as the cipher text and the decrypted data is known
as plain text. A key value is used for encrypting as well as decrypting data.
• Compression: Reduces the number of bits that need to be transmitted on the network.

10. Application Layer
• At the very top of the OSI Reference Model stack of layers, we find Application
layer which is implemented by the network applications.
• These applications produce the data, which has to be transferred over the
network. This layer also serves as a window for the application services to access
the network and for displaying the received information to the user.
• Ex: Application – Browsers, Skype Messenger etc.
• Application Layer is also called as Desktop Layer.
• Functions:
• Network Virtual Terminal
• FTAM-File transfer access and management
• Mail Services
• Directory Services